Communication systems typically include a plurality of communication devices, such as mobile or portable radio units, dispatch consoles and base stations (sometimes called base site repeaters) that are geographically distributed among various base sites and console sites. The radio units wirelessly communicate with the base stations and each other using radio frequency (RF) communication resources, and are often logically divided into various subgroups or talkgroups. Communication systems are often organized as trunked systems, where the RF communication resources are allocated on a call-by-call basis among multiple users or groups. Wide-area trunked systems are sometimes organized into a plurality of “zones,” wherein each zone includes multiple sites and a central controller or server (“zone controller”) for allocating communication resources among the multiple sites.
Next generation communication systems have begun to use Internet Protocol (IP) to transport packet data representative of voice, video, data or control traffic between endpoints (or “hosts” in IP terminology). In such systems, host devices, including base stations, consoles, zone controllers, and in some instances, wireless mobile or portable radio units in different zones, are logically interconnected by various routers forming an IP network. Data is divided into IP packets called datagrams, which include addressing information (e.g., source and destination addresses) that enables the routers of the network to transport the packets to the specified destination(s). The destination addresses may identify a particular host or may comprise an IP Multicast address shared by a group of hosts. Examples of communication systems using multicast addressing are described and claimed in U.S. Pat. No. 6,141,347, titled “Wireless Communication System Incorporating Multicast Addressing and Method for Use” and U.S. patent application Ser. No. 09/464,269, titled “Methods for Implementing a Talkgroup Call in a Multicast IP Network,” each of which is assigned to the assignee of the present invention and incorporated herein by reference in its entirety.
One of the underlying choices faced by IP multicast communication systems designers is that of which IP multicast routing protocol to use. Presently, there are two fundamental types of IP multicast routing protocols, commonly referred to as sparse mode and dense mode. Generally, in sparse mode, packets are communicated along a pre-established tree of router interfaces only to endpoints that have joined the multicast group address and hence, the sparse mode protocol is considered to provide an economical use of bandwidth and scale well for wide-area systems. In dense mode, the routers of the network employ a “flood-and-prune” operation whereby packets of a communication are initially distributed (or flooded) on all paths of the network. The distribution tree is then scaled back (or pruned) to eliminate unnecessary paths. Heretofore, the flood-and-prune approach of dense mode protocols was considered bandwidth wasteful and problematic in terms of scalability for wireless IP multicast communication systems, where participating radio units may roam between multiple sites and zones. However, dense mode protocols are known to exhibit improved join latency and decreased end-to-end delay relative to sparse mode. It would therefore be desirable to provide methods to limit the scope of flooding exhibited in dense mode IP multicast calls so as to make dense mode protocols an attractive alternative for wireless IP multicast communication systems. Advantageously, the method will provide for adjusting the scope of multicast group calls based on user location, as wireless users may roam from site to site or zone to zone. The present invention is directed to satisfying these needs.